1. Field of the Invention
The present invention relates to oxide superconductors and processes for preparing the same, and particularly to an oxide superconductor capable of exhibiting superconductivity within a wide range of composition and a process for preparing the same.
2. Description of the Prior Art
It is known that Nd--Ce--Cu--O oxides only falling within a very narrow range of composition can show superconductivity. For instance, Tokura et al. revealed that, among oxides represented by the composition formula: Nd.sub.2-a Ce.sub.a CuO.sub.4-d, only those within the composition range of 0.14.ltoreq.a.ltoreq.0.18 can exhibit superconductivity (Y. Tokura, H. Takagi and S. Uchida: Nature, Vol. 337, pp. 345-347, 1989).
Superconductors of Nd--Ce--Cu--O system further including Sr added thereto are also under study. For instance, Akimitsu et al. reported that Nd.sub.1.7 Sr.sub.0.4 Ce.sub.0.4 CuO.sub.y (Nd.sub.2 Sr.sub.0.5 Ce.sub.0.5 Cu.sub.1.2 O.sub.y) can exhibit superconductivity (J. Akimitsu et al.: Jpn. J. Appl. Phys., Vol. 27, No. 10, pp.L1859-1860, 1988), while Kosuge reported that oxides of Nd--Ce--Sr--Cu--O system can exhibit superconductivity within a very narrow range of composition, and that the superconducting transition temperature of (Nd.sub.0.638 Ce.sub.0.087 Sr.sub.0.275).sub.2 CuO.sub.4-d is 22 K. (M. Kosuge: Jpn. J. Appl. Phys., Vol. 28, No. 1, pp.L49-51, 1989).
As described above, since oxides of Nd--Ce--Cu--O system as well as Nd--Ce--Sr--Cu--O system can exhibit superconductivity only within the very narrow range of composition, there have arisen many problems with the preparation thereof. More specifically, powders of starting materials must be very precisely weighed in the preparatory step, while mixing of the starting material powders must sometimes be effected according to a complicated method such as spray drying. Furthermore, homogenization of a composition must be effected through alternately repeated calcination and pulverization in order to prevent the superconductivity of a sinter after heat treatment from lowering due to poor homogeneity of the composition inside the sinter. Thus, many steps are required in the preparation of such an oxide superconductor.